Widely used coated type magnetic recording media are typically prepared by dispersing magnetic oxide materials as magnetic particles, such as .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, Berthollide compounds composed of .gamma.-Fe.sub.2 O.sub.3, and Fe.sub.3 O.sub.4, Co-doped Berthollide compounds or CrO.sub.2 or magnetic materials of magnetic alloy particles mainly containing Fe, Co, or Ni in an organic binder such as copolymers of vinyl chloride and vinyl acetate, copolymers of styrene and butadiene, an epoxy resin or a polyurethane resin, coating the thus-prepared magnetic coating composition on a non-magnetic support, and drying it has widely been used.
Recently, with the increased demand for high density recording, attention has been drawn to a magnetic recoring medium of a ferromagnetic thin metal film type which is prepared by a vacuum evaporation method, a sputtering method, or an ion plating method without using a binder, and such non-binder type magnetic recording medium has been developed to realize it to a practical use.
Metal oxides having small saturation magnetization are conventionally used as a magnetic material for a coating type magnetic recording medium and the volume content of the magnetic material in a magnetic layer is only from 30 to 50%. Therefore, such magnetic recording medium is not suitable for high density recording with high output. Further, manufacturing steps thereof are complicated, and substantial auxiliary equipment is necessary for recovering used solvents and for preventing air pollution. On the other hand, a thin metal film type magnetic recording medium can be formed as an extremely thin film of ferromagnetic metal having a larger saturation magnetization than that of a magnetic oxide material, without containing non-magnetic materials such as an organic binder. With higher density recording, a magnetic head for recording and replaying having a gap length of 1.0 .mu.m or less has been used, and recording depth on a magnetic recording layer has had a tendency to be shallower. Therefore, a thin metal film type magnetic recording medium is the most suitable as a magnetic recording medium for high density recording with high output, because the whole thickness of the magnetic layer is utilized for recording magnetic signals. A thin film of a thin metal film type magnetic recording medium is formed by a vacuum evaporation method, because the vacuum evaporation method has advantages such as that rate for forming a film is fast, manufacturing processes are simple, and that this method is a dry process where treatment for waste solution is unnecessary. Among vacuum evaporation methods, a vacuum evaporation method wherein a vapor beam of a magnetic metal is vapor-deposited on a surface of a non-magnetic support by oblique incident evaporation method is practically the most suitable, because steps and apparatus thereof are comparatively simple, and a film having good magnetic properties can be obtained.
However, the magnetic recording medium comprising a ferromagnetic thin film prepared by a vacuum evaporation method has problems regarding an anti-corrosive property and surface-friction coefficient. Corrosion occurring with passage of time on a magnetic recording medium while it is stored leads to degradation and even erasure signals recorded on the magnetic recording medium and therefore corrosion should be prevented. Additionally, the friction coefficient on a surface of a magnetic recording medium must be low so that the magnetic recording medium has a smooth contact with a magnetic head or a guide.
Many methods have been proposed to improve the anti-corrosive property of a magnetic recording medium by vapor deposition; examples of such methods are disclosed in Japanese Patent Application (OPI) Nos. 198543/82 and 17544/83 (the term "OPI" as used herein means "published unexamined Japanese patent application"). These methods comprise forming a ferromagnetic thin film by a vapor deposition method on a surface of a non-magnetic base which is travelling along a periphery of a rotating drum, and then exposing a surface of the ferromagnetic thin film moving along the rotating drum to a glow discharge atmosphere using oxide gas. Decrease of residual magnetic flux density occurring while a magnetic recording medium is stored at high temperature and high humidity can be improved by this method, but weather resistance under conditions other than the above cannot always be improved. Furthermore, there is a problem with respect to the friction coefficient on a surface of a vapor deposited film.